Production of alkali metals from alkali amalgam



United States Patent 3,265,490 PRODUCTION OF ALKALI METALS FROM ALKALIAMALGAM Shiro Yoshizawa and Nobuatsu Watanabe, Sakyo-ku, Ky-

oto, and Tsukiro Morimoto, Masamichi Miura, and Yashuhiro Yamada,Toyama, Japan, assignors to Tekkosha Co., Ltd., Tokyo, Japan, acorporation of Japan Filed Apr. 9, 1963, Ser. No. 271,707

4 Claims. (Cl. 75-66) This invention relates .to a process, and to anelectrolytic bath for use in said process, for the production of alkalimetals by an amalgam electrolysis, and also to a process for thepurification of the thus obtained alkali metals.

As metallic sodium has desirable physical properties and is a strongreducing agent, it is finding increasing use for various purposes. Themelting point of metallic sodium is low MiP. 98 0.), its electricconductivity (s.r. 4.98x- Qcm, at 25 C.) and its thermal conductivity(0.170 caL/(cm (sec.) C./cm.) at 400 C.) are extremely good and itsspecific gravity (0.892 g./cc. at 250 C.) and its viscosity (0.381 c.p.at 250 C.) are low. Therefore, metallic sodium is very suitable for useas a heat .transfer medium and, as it can be used at high temperatures,it could find even greater use in many chemical apparatuses and heatexchangers if it were made available at a sufliciently low price. Also,although sodium is a very strong reducing agent, major uses of metallicsodium as a reducing agent have been limited because it is relativelyexpensive. Therefore, if its cost can be reduced, its use for .thispurpose will be increased.

The Castner process and the Downs process are commonly used for theproduction of metallic sodium. In the Castner process, metallic sodiumis produced by the fused-salt electrolysis of anhydrous sodiumhydroxide. The current elficiency of the process is usually about 40percent so that process is not economical. In the Downs process, fusedsodium chloride is electrolyzed directly and there are manydisadvantages due to the high temeratures which must 'be employed.

This invention provides a process and an apparatus for the electrolyticproduction of alkali metals, and also provides a process for thepurification of the metals, whereby alkali metal of high purity may beproduced relatively easily and economically.

According to this invention, alkali metals are produced from alkaliamalgams in an electrolytic cell. The cell consists of a first metallicplate serving as the anode and another metallic plate which serves asthe cathode. The anode plate can be a fiat plate disposed horizontallyor slightly inclined, or it can be conically shaped. The anode plate isrotated slowly and the alkali metal amalgam from a chlorine cell isflowed onto the center of the anode plate and it flows to the edge ofthe anode plate by centrifugal force. A fused salt is used as theelectrolyte in the cell.

According to this invention, the amalgan is flowed forcibly onto theanode plate in such fashion that the rate of flow can be adjusted asdesired. It is an advantage of the invention that the process can beoperated at a low voltage. Also, this invention is advantageous in thatthe metal deposited at the cathode is not contami- 3,265,490 PatentedAugust 9, 1966 nated by the amalgam as it is in the case withverticaltype cells. Therefore, the mercury content of the depositedmetal is relatively low. The current efficiency is very high because thedeposited metal floats quickly to the surface of the electrolyte, andthe operating temperature is lower. Also, the anode plate is alwayscovered with the amalgam and thus, the anode plate is not oxidized.Therefore, the anode plate can be made of iron instead of nickel.

In addition to the advantages and merits mentioned above, the cost ofthe apparatus is lower than with other procedures, the operation can becarried out easily and smoothly, and the current efficiency isremarkably high, so that alkali metals of high purity can be produced ata markedly reduced cost.

The invention is diagrammatically illustrated by way of example, in theattached drawing, in which:

FIGURE 1 is a central sectional view of an electrolytic cell used forproducing alkali metal according to this invention;

FIGURE 2 is the equilibrium diagram near the eutectic point of the threecomponents of the electrolyte used in this invention consisting ofsodium hydroxide, sodium iodide and sodium cyanide; and

FIGURE 3 is the equilibrium diagram near the eutectic point of aconventional three component electrolyte consisting of sodium hydroxide,sodium iodide and sodium bromide.

Referring to FIGURE 1, the cell comprises a horizontal, iron disc 1which is supported for rotation by a hollow shaft 2 which extendsdownwardly from the lower surface of said disc at the center thereof.

The shaft 2 can be rotated at any suitable speed by means of anyconventional drive system, such as a motor and gear box arrangement (notshown). In this example, the upper surface of the rotary iron disc 1 ishorizontal and flat, but it may be conical or inclined. 'Further, theupper surface of disc 1 is not necessarily flat, and it may be providedwith suitable grooves, projections, teeth or agitating means. Further,it is not necessarily of disc-form. Thus, the apparatus can be modifiedsuitably within the scope of this invention.

The central opening 3 in the rotary shaft 2 serves as an amalgamtransport pipe through which the amalgam is supplied from a suitablesource of supply, such as a mercury cell, to the upper surface of therotary iron disc 1. However, the amalgan supplying conduit may be placedat other places in the apparatus so that the amalgam can be supplied tothe upper surface of the disc 1 from above or from the side of saiddisc. As shown in FIGURE 1, the amalgam is flowed directly onto thesurface of the iron disc, but it may be flowed into a circular recess oran annular weir on the upper surface of or at the central portion of theiron disc and then the amalgam may overflow onto the upper surface ofsaid disc.

An amalgam discharging pipe 5 is connected to the bottom wall 14 of thecell adjacent the periphery of the disc 1. The other end of the pipe 5is connected to an amalgam level controlling chamber (not shown) placedoutside the electrolytic cell. Any suitable type of level controlapparatus can be used for controlling the level of the amalgam in thecell. For example, an annular, dilute amalgam reservoir can be placed atthe periphery of the bottom plate 4 of the electrolytic cell, theperipheral region of the rotary iron disc 1 be bent down, and the bentregion is immersed in the dilute amalgam reservoir, whereby the regionis sealed by the amalgam to prevent leakage and the amalgam isdischarged at a suitable place from the amalgan reservoir. Heatingjackets 6 are placed at the bottom and around the sides of the cell sothat the operating temperature of the cell can be maintained at adefinite temperature by a heating medium, or a hot gas, or an electricheater in the jacket.

The stationary cathode plate 7 is of the same shape as the rotary irondisc 1 and is placed thereabove with its lower surface face to facewith, and spaced vertically from, the upper surface of the rotary irondisc 1. The cathode plate 7 is supported on a cover 8 by supports 8a.The plate 7 may be made of a suitable metal, such as iron. The cathodeplate 7 is shown in FIGURE 1 as being a relatively thick plate, but itmay be thin or screenlike.

A hydrogen inlet pipe 9 extends through the cover 8 of the cell, and theend of said pipe is placed between the cathode plate 7 and the anodeplate 1 adjacent the periphery thereof. Hydrogen is used to prevent theoxidation of deposited sodium (Na O-j- /zH Na-l-NaoH). Only one hydrogeninlet pipe is illustrated in FIGURE 1, but two or more such pipes can beused according to the capacity of the cell. A scraper device 13 ispositioned between the rotary anode plate 1 and the cathode plate 7 andit is rotated by a rotary shaft 14 which extends through the cell cover8 and the central opening in the cathode plate 7. The scraper device 13is near the cathode plate 7 and the deposited metal on the cathode plate7 is quickly scraped off by the blades thereon so that the formation ofsodium oxide is minimized.

The cell is filled to a level above the upper surface of cathode plate 7with an electrolyte. Since the electrolyte is brought into contact withhydrogen gas by the agitation effected by the scraper device 13 and thedisc 1, the viscosity of the electrolyte does not increase, the metalcan be deposited smoothly and, thus, an increase of the electrolyticvoltage and a reduction of the current efiiciency can be prevented sothat the electrolysis can be continued at a high efiiciency for a longtime.

The level of the electrolyte is usually maintained above the uppersurface of the cathode plate 7. When the cathode 7 is of disc-form,slots 10 or many perforations are provided in it through which the metalrises up to the upper surface of the electrolyte by its bouyancy. Themetal on the surface of the bath is withdrawn from the cell through ametal discharging pipe 11 which extends through the wall of the cellnear the surface of the bath. A hydrogen discharging pipe 12 is weldedto the cover 8 of the cell.

It will be understood that various conventional devices, such aselectrical connections to the anode plate 1 and cathode plate 7, variousbearing and seal units, etc., will be provided but these have beenomitted from the drawing because they are conventional.

The electrolyte bath composition according to this invention contributeseffectively to enable metallic sodium of high purity to be producedeasily and economically. In conventional processes, a fused-salt bath ofa NaOH-NaI-NaBr mixed-salt system has been used. Electrolysis using thismixed-salt system, however, must be carried out at a high temperature,e.g., 230250 C. Further, since the amalgam supplied has a relativelylarge vapor pressure of mercury at such high temperatures, the depositedmetal contains a small amount of mercury corresponding to the vaporpressure of mercury. Therefore, in order to reduce the mercury contentin the deposited metal, it is necessary to reduce the mercury vaporpressure of the amalgam by lowering the operating temperature as much aspossible. Also, the resolution and diffusion of the deposited metal intothe bath can be lowered by reducing the operating temperature so thatthe current efliciency can be higher. However, as the eutectic point ofthe NaOH-NaI-NaBr bath is 215 C., it is practically impossible tooperate systems employing such a bath at a temperature lower than 230 C.

According to our knowledge and experiments, bath compositions which areacceptable for the purposes of our invention, are very limited. Forexample, since in a fused-salt bath containing different kinds ofcations, such as potassium and calcium, electrolytic deposition of allthe cations occurs, only sodium cations can be used in the bath of ourinvention to produce sodium metal. Moreover, although a salt, such asNaNO has a comparatively low melting point (M.P. 308 C.), it reactsviolently with deposited metallic sodium. Also, a fusedsalt having a lowspecific gravity can not be used because it disturbs the floating of thedeposited metal. Also, sodium salts of a low electric conductivity andhigh viscosity are not suitable.

We have discovered that a novel fused mixed-salt bath consisting ofanhydrous salts of NaOH, NaI, and NaCN in most suitable for use as thebath for the production of sodium metal from an amalgam. The meltingpoint and the viscosity of such a bath are low and its electricconductivity and density are high, so that it is particularlyadvantageous for the production of the alkali metal from the amalgam.Also, such a bath is not corrosive to the cell.

The equilibrium diagram of a NaOH, NaI and NaCN mixed-salt bath is shownin FIGURE 2. We have found it possible to use a bath having 4070% byweight sodium hydroxide, less than 50 weight percent sodium iodide andless than 40 weight percent sodium cyanide, all of such percentagesbeing in such proportions as to be encompassed within the 230 C.isotherm of FIGURE 2 of the accompanying drawing. The operatingtemperature can be remarkably lowered compared with the conventionalelectrolytic bath employing a NaOH-NaI-NaBr system. Thus, the operationcan be carried out at from 210 to 230 C., which is 20 C. lower than thetemperature used with the conventional electrolytic bath. Therefore, inaccordance with this invention, the current efficiency will be higherand the mercury content in the deposited metal will be lower.

Further, as is clearly understood by comparing the equilibrium diagramof the conventional NaOH-NaI-NaBr system shown in FIGURE 3 with theequilibrium diagram of our system shown in FIGURE 2, the electrolyticbath of this invention has the feature that the region enclosed by agiven isotherm is several times larger than that in the correspondingisotherm in the conventional system. The sodium hydroxide component inthe 220 C. isotherm ranges from 43 to 65 weight percent in the diagramof FIGURE 2, and, on the other hand, it ranges from 47 to 51 weightpercent in the diagram of FIGURE 3. The sodium hydroxide concentrationgradually increases during the electrolysis due to the water in amalgam.Even if the composition ratio of the electrolytic bath according to ourinvention changes through a considerably broader range than is the casewith the conventional electrolytic bath, the bath temperature still canbe maintained lower. This is another advantage of the bath of thepresent invention.

Since, in an amalgam electrolysis, the deposited metal reacts with thesodium hydroxide in the bath to form sod um oxide, which is thenconverted with hydrogen into sodium hydroxide, the bath compositionchanges during the operation. The bath of this invention has, however, astrong resistance to this composition change.

Thus, the NaOH-NaI-NaCN electrolytic bath of the present invention ismost suitable as the electrolytic bath to be used for the production ofalkali metal from an amalgam. If desired, small amounts of other sodiumsalts, such as NaCl, NaBr and the like, may be added to the mixed-saltbath of the NaOH-NaI-NaCN three component system.

However, even with the improved amalgam electrolysis process of thisinvention, the contamination of the deposited metal with mercury is notavoidable and it is desirable to remove as much of the mercury from thedeposited metal as possible. In accordance with the purification featureof this invention, this can be carried out effectively and at a lowcost, which makes a very important contribution from an industrialviewpoint.

In the purification processes hitherto reported, the deposited sodiumcontaminated by mercury has been purified with metallic calcium at about380 C. whereby mercury is removed as a calcium amalgam. By this processthe mercury content in the sodium metal can be reduced to about 0.01percent. However, expensive metallic calcium is needed in an amountalmost equal to that of the mercury in the deposited sodium.

We have found that mercury can be removed from the deposited metal byusing metallic magnesium instead of metallic calcium. For example, whenthe contaminated metal containing mercury (the mercury content is about1 percent) is passed through an apparatus packed with metallicmagnesium, in an amount almost the same as that of the mercury containedin the sodium, at 350 to 400 C., up to 99.9 percent of the mercury canbe removed easily from the metal.

According to the purification process of this invention, metallicmagnesium is added into the molten deposited metal at a temperaturehigher than 400 C. and is stirred, and then the mixed metal is allowedto settle at 100200 C. for one or two hours, and then the pure sodiummetal is separated from the other metals. In the purification process, asmall amount of other metals may be used together with magnesium, ormagnesium alloy may be used. By this purification process, metallicsodium having a mercury content lower than 0.01 percent and usually of0.002 to 0.005 percent can be obtained.

Example I An electrolysis was carried out using a horizontal type cellwith a rotary anode disc as shown in FIGURE 1 (the diameter of therotary iron disc was 140 cm., its speed of rotation r.p.m., and thespeed of rotation of the scraper blade 10 r.p.m.) and using anelectrolytic bath having a composition of 47 weight percent of NaOH, 36weight percent of NaI and 17 weight percent of NaCN. The sodiumconcentration of the supplied sodium amalgam was 0.2 to 0.3 percent, thefeeding rate of the amalgam was 45 kg./min., the cell voltage was 2.0 to2.3 volts, the anode current density was 39 to 52 amp/cm. (the electriccurrent was from 6,000 to 7,900 amp), and the electrolysis temperaturewas 210 to 230 C. After 50 hours, 282 kg. of metallic sodium having amercury content of 0.7 percent was obtained with a current efficiency of92 percent.

Example 11 To 1 kg. of the metallic sodium containing 0.7 percent ofmercury obtained as stated in Example I 7 g. of metallic magnesium wasadded and the mixture was heated at 450 C. with agitation in a reactorfor one hour. The mixture was then cooled to l20200 C., allowed to besettled and then metallic sodium was separated. The mercury content ofthe metallic sodium was 0.002 percent.

As mentioned above in detail, as the amalgam is pumped or otherwiseforcibly flowed into the cell the supplied amount of the amalgam can beincreased remarkably whereby the electrolysis can be operated at a highcurrent density. When the pure metallic sodium is produced from anamalgam by using the process of the invention in an industrial-scalecell (of a capacity of more than 30,000 amp.), it should be expectedthat the current efliciency of 96 to 98 percent can be obtained.

What is claimed is:

1. An electrolytic bath for the electrolytic production of alkali metal,comprising a three-component, fused, mixed salt consisting essentiallyof sodium hydroxide, sodium iodide and sodium cyanide, the sodiumhydroxide being present in an amount of 4070% by weight, the sodiumiodide being present in an amount of less than 50% by weight and thesodium cyanide being present in an amount of less than 40% by weight,all of such percentages being in such proportions as to be encompassedwithin the 230 C. isotherm of FIGURE 2 of the accompanying drawing.

2. A process for producing alkali metals by an amalgam electrolysis,using an electrolytic cell comprising an anode plate having asubstantially horizontal upper surface and a cathode disposed above andspaced from said upper surface, which process comprises:

continuously flowing a fluid alkali metal amalgam onto said uppersurface to forma moving layer thereon; maintaining a fused, mixed salt,electrolytic bath in said cell at a level at least as high as saidcathode; electrically energizing said cathode and said layer so thatsaid layer functions as an anode whereby the alkali metal is depositedon said cathode; flowing hydrogen gas into the space between said anodeplate and said cathode so that said hydrogen gas contacts theelectrolyte and hinders an increase in the viscosity of the electrolyte;and collecting the alkali metal deposited on said cathode. 3. A processfor producing alkali metals by an amalgam electrolysis, using anelectrolytic cell comprising an anode plate having a substantiallyhorizontal upper surface and a cathode disposed above and spaced fromsaid upper surface, which process comprises:

continuously flowing a fluid alkali metal amalgam onto said uppersurface to form a moving layer thereon;

maintaining a fused, mixed salt, electrolytic bath in said cell at alevel at least as high as said cathode, said bath consisting essentiallyof sodium hydroxide, sodium iodide and sodium cyanide, the sodiumhydroxide being present in an amount of 4070% by weight, the sodiumiodide being present in an amount of less than 50% by weight and thesodium cyanide being present in an amount of less than 40% by weight,all of such percentages being in such proportions as to be encompassedwithin the 230 C. isotherm of FIGURE 2 of the accompanying drawing;

electrically energizing said cathode and said layer so that said layerfunctions as an anode whereby the alkali metal is deposited on saidcathode; and collecting the alkali metal deposited on said cathode. 4. Aprocess for producing alkali metals by an amalgam electrolysis, using anelectrolytic cell comprising an anode plate having a substantiallyhorizontal upper surface and a cathode disposed above and spaced fromsaid upper surface, which process comprises:

continuously flowing a fluid alkali metal amalgam onto said uppersurface to form a moving layer thereon;

maintaining a fused, mixed salt, electrolytic bath in said cell at alevel at least as high as said cathode, said bath consisting essentiallyof sodium hydroxide, sodium iodide and sodium cyanide, the sodiumhydroxide being present in an amount of 4070% by weight, the sodiumiodide being present in an amount of less than 50% by weight and thesodium cyanide being present in an amount of less than 40% by weight,all of such percentages being in such proportions as to be encompassedwithin the 230 C. isotherm of FIGURE 2 of the accompanying drawing;

electrically energizing said cathode and said layer so that said layerfunctions as an anode and the alkali metal is deposited on said cathode;

collecting the alkali metal deposited on said cathode;

melting the thus obtained alkali metal;

2,054,316 9/1936 2,124,564 7/1938 Gilbert 75-63 2,234,967 3/ 1941Gilbert 20468 mixing metallic magnesium into the molten alkali metal2,745,552 at a temperature in excess of between 400 C.; 2,916,425cooling the solution to a temperature of between 100- 200 C.; allowingthe mixture to settle; and 5 635,747 then separating the purified alkalimetal. 1 0,980

References Cited by the Examiner UNITED STATES PATENTS 8 5/ 1956Bruggeman 75-66 12/1959 Fujioka 204-68 FOREIGN PATENTS 1/ 1962 Canada.

1904 Great Britain.

1914 Great Britain.

DAVID L. RECK, Primary Examiner.

Gilbert 75 66 10 BENJAMIN HENKIN, Examiner.

H. w. CUMMINGS, H. WiTARRING,

Assistant Examiners.

4. A PROCESS FOR PRODUCING ALKALI METALS BY AN AMALGAM ELECTROLYSIS,USING AN ELECTROLYTIC CELL COMPRISING AN ANODE PLATE HAVING ASUBSTANTIALLY HORIZONTAL UPPER SURFACE AND A CATHODE DISPOSED ABOVE ANDSPACED FROM SAID UPPER SURFACE, WHICH PROCESS COMPRISES: CONTINUOUSLYFLOWING A FLUID ALKALI METAL AMALGAM ONTO SAID UPPER SURFACE TO FORM AMOVING LAYER THEREON; MAINTAINING A FUSED, MIXED SALT, ELECTROLYTIC BATHIN SAID CELL AT A LEVEL AT LEAST AS HIGH AS SAID CATHODE, SAID BATHCONSISTING ESSENTIALLY OF SODIUM HYDROXIDE, SODIUM IODIDE AND SODIUMCYANIDE, THE SODIUM HYDROXIDE BEING PRESENT IN AN AMOUNT OF 40-70% BYWEIGHT, THE SODIUM IODIDE BEING PRESENT IN AN AMOUNT OF LESS THAN 50% BYWEIGHT AND THE SODIUM CYANIDE BEING PRESENT IN AN AMOUNT OF LESS THAN40% BY WEIGHT, ALL OF SUCH PERCENTAGES BEING IN SUCH PROPORTIONS AS TOBE ENCOMPASSED WITHIN THE 230*C. ISOTHERM OF FIGURE 2 OF THEACCOMPANYING DRAWING; ELECTRICALLY ENERGIZING SAID CATHODE AND SAIDLAYER SO THAT SAID LAYER FUNCTIONS AS AN ANODE AND THE ALKALI METAL ISDEPOSITED ON SAID CATHODE; COLLECTING THE ALKALI METAL DEPOSITED ON SAIDCATHODE; MELTING THE THUS OBTAINED ALKALI METAL; MIXING METALLICMAGNESIUM INTO THE MOLTEN ALKALI METAL AT A TEMPERATURE IN EXCESS OFBETWEEN 400*C.; COOLING THE SOLUTION TO A TEMPERATURE OF BETWEEN100200*C.; ALLOWING THE MIXTURE TO SETTLE; AND THEN SEPARATING THEPURIFIED ALKALI METAL.